1. Field of the Invention
This invention relates to magnetically soft alloy powder suitable for forming magnetic shields, a magnetic shielding composition containing the same, and a process of making the same.
2. Prior Art
Magnetic shields are used to prevent magnetized objects or magnetic field-producing sources from affecting other objects, electric circuits or the like. Because of their high magnetic permeability, metal plates are usually good magnetic shields. However, the metal plates have a limited range of applications. Metal powder is more convenient because magnetic shields of any desired shape can be formed therefrom at any desired location. Powder may be dispersed in an organic binder to form a coating composition which may be applied to a site to be shielded or coated onto a suitable flexible substrate to form a magnetic shield.
A variety of magnetic shielding compositions have been proposed which contain high magnetic permeability metal powder. For example, Japanese Patent Application Kokai No. 58-59268 discloses a magnetic shielding coating composition comprising flat particles of high magnetic permeability alloy and a polymeric binder. Japanese Patent Application Kokai No. 59-201493 discloses a magnetic shielding coating composition comprising flat particles of magnetically soft amorphous alloy and a polymeric binder. These compositions comprising flat particles show isotropic magnetic shielding characteristics between planes perpendicular to a thickness direction when applied as a film.
The flat particles of magnetically soft amorphous alloy disclosed in Japanese Patent Application Kokai No. 59-201493 are produced by melting a transition metal-metalloid alloy at a high temperature, contacting the melt to the surface of a chill roll to rapidly quench the alloy, and crushing the resulting ribbon. In general, there are available ribbons having a thickness of about 10 to about 50 .mu.m because the process imposes a lower limit of 10 .mu.m upon the ribbon thickness. Such ribbons are crushed into a scaly powder, in which the thickness of scales remains unchanged. The scaly powder is then blended with a binder to form a coating which has poor magnetic shielding properties. Although amorphous alloy itself has very good magnetic properties, scaly powder thereof cannot take full advantage of its own nature.
One method for preparing scales by rapidly quenching alloy and crushing is disclosed in Japanese Patent Application Kokai No. 58-197205. Crushing is carried out by conventional well-known means such as stamp mills, dry ball mills, wet ball mills, attritors, and vibratory mills. Japanese Patent Application Kokai No. 60-401 discloses the preparation of scales by rapidly quenching amorphous alloy in sheet, ribbon, tape or wire form, rendering the ribbon alloy brittle at a temperature lower than its glass transition temperature, and crushing the ribbon. Also the alloy is crushed by such means as rod mills, ball mills, impact mills, disk mills, stamp mills, and crusher rolls. These crushing means fail to comminute the given material to a size smaller than the minimum thickness of sheet or ribbon. There are produced isotropic scales. This is because the conventional crushing techniques have little possibility of cleaving alloy grains or rolling them into thinner ones. A coating composition containing such scales of a relatively large size is difficult to form an even coating. A magnetic shield formed therefrom tends to be uneven in magnetic shielding performance, allowing local leakage of a magnetic field.
Among magnetic fields to be shielded, some are isotropic, but many are more intense in a particular direction. In the latter case, it is desired to design the shield so as to achieve an intense magnetic shielding effect in the particular direction. A shield prepared from conventional scaly powder is little effective even when scales are magnetically oriented during coating.
When amorphous alloy was milled in a vibratory ball mill, which is one of the mills most widely used with amorphous alloys, to determine the relationship of average outer diameter (D50 to be defined later) of particles to the milling time, it was found that the average outer diameter D50 decreases with time. The average outer diameter eventually reaches to the range of about 1 to about 30 .mu.m, although the actual value varies somewhat with milling parameters, and milling is promoted no longer. The particles produced by milling are initially flat in shape and gradually becomes granular or round with the lapse of milling time. It was observed that milling includes two stages. In the first stage of milling, amorphous ribbons are crushed into relatively flat shreds having an average outer diameter of about 50 .mu.m to several mm and a thickness of about 10 to about 50 .mu.m while little crushing takes place in a thickness direction of the ribbon. In the second stage of milling, relatively flat shreds are further crushed into uneven irregular granular particulates. The powder known as comprising flat shreds is a powder obtained from the first stage of milling. The powders described in the above-cited publications also belong to this class. It is difficult to prepare a coating composition from such flat shreds which is as thick as 10 to 50 .mu.m. On the other hand, granular particles obtained in the second stage, particularly fine particles having a diameter of several .mu.m are suitable to form a coating composition, but have poor magnetic shielding properties because of isotropy.
Another important problem associated with magnetic shields is corrosion resistance. Conventional shields of metal or alloy often rust when used at elevated temperatures and high humidity. There is a need for a corrosion resistant magnetic shield.